Very low blood flow, or low "systemic perfusion," is typically due to low aortic pressure and can be caused by a number of factors, including hemorrhage, sepsis and cardiac arrest. When there is a reduced flow of blood from the heart, the body directs a higher portion of blood to critical organs, such as the brain, which will not survive long without a continuous supply of blood, while restricting the flow to less critical organs, such as the stomach and intestines, whose survival is not as threatened by a temporary large reduction in blood flow. Physicians commonly take advantage of this phenomenon by taking measurements in the stomach and intestine to assess perfusion failure.
Assessment of CO.sub.2 concentration in the less critical organs, i.e., those organs to which blood flow is reduced during perfusion failure, is useful in perfusion assessment. Carbon dioxide production, which is associated with metabolism, continues even during low blood flow. Because CO.sub.2 is not rapidly carried away during low blood flow, the concentration of CO.sub.2 increases, which in turn results in a decrease in pH and an increase in partial pressure of CO.sub.2 (pCO.sub.2) in the less critical organs. Therefore, perfusion failure is commonly assessed by measuring pH or pCO.sub.2 at these sites, especially in the stomach and intestines. For examples of catheters used to assess pH or pCO.sub.2 in the stomach or intestines, see, e.g., U.S. Pat. Nos. 3,905,889; 4,016,863; 4,632,119; 4,643,192; 4,981,470; 5,105,812; 5,117,827; 5,174,290; 5,341,803; 5,411,022; 5,423,320; 5,456,251; and 5,788,631.
The measurement of pCO.sub.2 to determine the extent of perfusion failure has commonly been done by threading a catheter through the nasal passage, past the epiglottis, through the esophagus, past the esophageal sphincter, and into the stomach, and sometimes through the stomach and into the intestines. Alternatively, measurement has been conducted in the colon, with a catheter being threaded through the anus. These procedures are obviously quite invasive and can cause harm and discomfort to a patient. Moreover, insertion of the catheter in this manner is also complex and time-consuming.
In U.S. Pat. No. 5,579,763, applicants described the introduction of a catheter with a carbon dioxide sensor through the nasal or oral passage, past the epiglottis, and into the esophagus so that the catheter and sensor lay within the esophagus. This method can be used to accurately assess perfusion failure by measuring pCO.sub.2 in the patient's esophagus of a patient, rather than in the stomach and/or intestine. Tests showed that measurements of pCO.sub.2 in the esophagus are closely correlated with aortic pressure, and, furthermore, that measurements made in the esophagus are even more closely correlated to aortic pressure than measurements of CO.sub.2 in the stomach. This procedure was advantageous in that the procedure's invasiveness was reduced and CO.sub.2 generated by digestive fluids in the stomach did not affect measurements since the esophageal sphincter blocks such gas. However, the insertion of the catheter still constituted considerable invasion and thus risk of harm to the patient. Furthermore, extension of the catheter extended past the epiglottis exposed the patient to the risk of regurgitation of stomach contents including stomach acids.
There is a need for an even less invasive method to measure perfusion failure and to monitor the effectiveness of methods taken to increase perfusion, e.g., blood infusion or the like.